1. Field of the Invention
The invention relates to an arrangement for equalizing and demodulating a binary continuous-phase angle-modulated data signal with a modulation index h=0.5 which includes a baseband converter for producing both quadrature components of a received modulated data signal in the baseband, and an equalizer/detector to which both baseband quadrature components are applied for recovering the original binary data signal.
2. Description of the Related Art
In information transmission systems comprising dispersive transmission channels the signal to be transmitted is distorted by delay differences and phase shifts. In digital information transmission these distortions become apparent as intersymbol interference in the received signal. In addition, the received signal may be affected by noise or similar interference signals. In order to recover the original data content of the signal, it is necessary to equalize the received signal.
Equalizers for data signals with linear modulation, such as, for example, Pulse Amplitude Modulation (PAM) or Quadrature Amplitude Modulation (QAM), are very extensively investigated and described in the literature. Their technical realization is usually possible with acceptable cost and design efforts. This is quite different for equalizers for data signals with non-linear angle modulation, such as, for example, a frequency modulation with continuous-phase variation, which is denoted as Continuous Phase Modulation (CPM). The equalization methods and arrangements for CPM-signals, as disclosed in the literature, are generally very complex and expensive and therefore rather unsuitable for an advantageously priced technical realization. This holds more specifically for CPM-modulation types using a Partial-Response method, that is to say, in which the signal variation of the modulated signal does not only depend on the instantaneous data bit to be transmitted, but in a defined manner also on a fixed number of data bits immediately preceding this data bit. Examples of important binary Partial-Response-CPM modulation methods are the Gaussian Minimum Shift-Keying (GMSK) and Generalized Tamed Frequency Modulation (GTFM).
An arrangement for equalizing and demodulating a Partial-Response-CPM signal is known from, for example, the article "On Optimum and Suboptimum Coherent Detection of Continuous Phase Modulation on a Two-Ray Multipath Fading Channel", N. A. B. Svensson, IEEE Transactions on Communications, Vol. COM-35, No. 10, pages 1041-1049, Oct. 1987. This arrangement is based on a demodulator for distortion-free received Partial-Response-CPM signals, disclosed in the article "Continuous Phase Modulation", C.-E. Sundberg, IEEE Communications Magazine, Vol. 24, No. 4, pages 25-38, Apr. 1986. FIG. 10, page 31 of the latter publication shows a quadrature receiver which is composed of a baseband converter and a Viterbi detector. The baseband converter comprises a bandpass filter to which the input signal is applied and two mixers, each of which receives the bandpass-filtered signal and each of which is followed by a low-pass filter. The mixing carrier signal of the first mixer has, compared with the mixing carrier signal of the second mixer, a phase which lags by 90.degree., the frequency of the mixing carrier signals corresponding to the carrier frequency of the received modulated signal. The low-pass filters arranged subsequent to the mixers only pass the low-frequency signal part of the mixing product. The low-pass-filtered output signal of the first mixer is commonly referred to as the in-phase component (I-component), the low-pass-filtered output signal of the second mixer is commonly referred to as the quadrature-phase component (Q-component). These two quadrature components are applied to a Viterbi detector, which recovers the transmitted data in accordance with the Maximum Likelihood Sequence Estimation method (MLSE). In the quadrature receiver for distorted CPM-signals in accordance with Svensson's paper, the Viterbi detector is replaced by a Viterbi equalizer (see FIG. 2, page 1043 of said paper), which also operates in accordance with the MLSE-method. At the same time it performs the equalization, the Viterbi equalizer effects the detection, i.e. the recovery of the original data to be transmitted. It then takes account of both the structure, the Partial-Response property of the CPM-modulation and the characteristics of the distorting transmission channel. Basically, other types of equalizers such as, for example, decision-feedback equalizers can also be used instead of the Viterbi equalizer, c.f. said publication by Svensson.
The fact that the CPM-modulation is a non-linear angle modulation and that in addition to the linear intersymbol interference by the dispersive transmission channel also non-linear intersymbol interference by the Partial-Response property of the modulation occurs, is the reason that equalizers for CPM-signals are significantly more complex and more expensive than equalizers for linearly modulated data signals such as PAM-signals or QAM-signals. From Svensson's paper it is evident that both the special structure of the phase-continuous frequency modulation, the non-linear partial-response characteristic of the modulation, and also the linear intersymbol interference by the dispersive transmission channel must be taken into account in a complicated manner in the equalizer. This requires the real-time execution of a very large number of extensive multiplications and additions of complex quantities.